1. Field of the Invention
The present invention relates to a spark plug used as an ignition device for an internal combustion engine. More particularly, the invention relates to a spark plug that employs a chip or member resistant to spark consumption, made of an iridium-based alloy having a high melting point.
2. Description of the Related Art
Conventionally, the electrode of a standard spark plug has been made of a nickel-based alloy having corrosion resistance and a high melting point, specifically, an alloy containing Ni in an amount of 90% or more. In a spark plug required to exhibit high performance and long life, its spark portion is formed such that iconel is used as a base material and a noble metal chip having a melting point higher than that of inconel is welded to the base material. A noble metal chip of platinum or platinum-based alloy has been of practical use. A spark plug having such a noble metal chip is called a platinum spark plug.
Recently, with growing demand for higher performance (improved igniting performance) and longer life (improved resistance to spark consumption), studies have been carried out on a spark plug that employs a chip made of an alloy of iridium, which has a melting point higher than that of platinum.
Meanwhile, increases in engine output and increasing use of spark plugs in direct-injection engines involve a significant increase in thermal load exerted on the chip of a spark plug. Since iridium oxide shows volatility at high temperature, in some cases, even a chip of a highly durable iridium alloy has exhibited drastic, unusual consumption or biased consumption, conceivably due to increased chip temperature. Also, a core rod made of inconel and carrying the chip is heated excessively and a resultant thermal deformation thereof has occasionally caused cracking in an insulator.
The present inventors have carried out various experiments and have found that when dimensional restrictions are imposed on a chip made of an iridium alloy and a core rod made of inconel and supporting the chip, unusual consumption or biased consumption of the chip can be avoided and excessive heating of the core rod can be prevented.
In view of the foregoing, an object of the present invention is to provide a spark plug employing a chip of iridium and capable of avoiding unusual consumption or biased consumption of the chip and suppressing excessive heating of a core rod.
To achieve the above object, the present invention provides a spark plug comprising an insulator having a center through-hole formed therein; a center electrode held in the center through-hole; a metallic shell for holding the insulator; and a ground electrode electrically connected to the metallic shell. The center electrode comprises a core rod and a chip. The core rod is made of a nickel-based alloy and includes a copper core embedded therein. The chip is formed of a member resistant to spark consumption and fixed to a tip end of the core rod in order to form a spark discharge gap in cooperation with the ground electrode. The chip is made of an iridium-based alloy which contains one or more elements selected from among platinum, rhodium, rhenium, and palladium. In this specification the term xe2x80x9calloyxe2x80x9d means alloy which is formed through melting of its base metal and additional metal elements. The distance between the chip and a tip end of the copper core is not greater than 3.5 mm; the diameter of the core rod is not less than 1.4 mm but not greater than 2.6 mm; and D2/D1xe2x89xa60.50, where D1 is the diameter of the core rod and D2 is the diameter of a spark discharge gap portion of the chip (hereinafter simply referred to as the xe2x80x9cdiameter of the chipxe2x80x9d).
Since the distance between the chip and the copper core having high thermal conductivity is not greater than 3.5 mm, the temperature of the chip decreases through good heat conduction to the copper core. Although the definite reason is unknown, employment of a D2/D1 ratio of not greater than 0.5 yields the effect of suppressing drastic, unusual consumption of the chip and excessive heating of the core rod.
Employment of the above-mentioned iridium-based alloy, not pure iridium, as material for the chip yields the effect of suppressing volatilization of iridium at high temperature, which would otherwise occur in the form of oxidation, thereby suppressing unusual consumption of the chip. Especially, unusual consumption of the chip easily occurs when the content of platinum, rhodium, rhenium and/or palladium is not less than 25 wt. % but less than 50 wt. %. In this case the effect obtained through employment of the above-described dimensions becomes remarkable.
Preferably, the diameter D1 of the core rod is not greater than 2.3 mm, and the D2/D1 ratio is not greater than 0.45.
In this case, even when the diameter D1 of the core rod is not greater than 2.3 mm, unusual consumption of the chip can be suppressed.
Preferably, the diameter D1 of the core rod is less than 2.0 mm, and the D2/D1 ratio is not greater than 0.40.
In this case, even when the diameter D1 of the core rod is less than 2.0 mm, unusual consumption of the chip can be suppressed. As the diameter D1 of the core rod decreases, the D2/D1 ratio that yields the effect of suppressing unusual consumption of the chip decreases. This is conceivably because, as the diameter D1 of the core rod decreases, heat conduction from the chip to the core rod deteriorates.
Preferably, the diameter D2 of the chip and an axially exposed length L of the chip satisfy the relationship xe2x80x9cLxe2x89xa61.2xc3x97D2.xe2x80x9d
Employment of the above dimensional relationship yields the effect of suppressing biased consumption of the chip. Conceivably, when the axial length of the chip becomes too long, heat conduction from the chip to the core rod deteriorates. As a result, the temperature of a portion of the chip increases to an unusually high level, thereby causing oxidation and evaporation of iridium, with resultant biased consumption of the chip. Therefore, employment of the above restriction on the axially exposed length L of the chip yields the effect of suppressing biased consumption of the chip.
The spark plug of the invention is equipped with the chip of the iridium-based alloy and imposes the predetermined restriction on the relationship between the chip diameter and the core rod diameter, thereby preventing unusual consumption or biased consumption of the chip and suppressing excessive heating of the core rod. Thus, the spark plug of the invention facilitates the feature of iridium that the melting point is high and provides high performance and long life.